Measurement of $BR(K^{+} \to \mu^{\pm}\nu_{\mu}e^{+}e^-$ with NA48/2 at CERN

The exclusive branching ratio of the radiative decay is measured using data collected by the NA48/2 experiment at CERN in 2003-2004. The measurement is performed in the phase-space region of the mass of the electron-positron pair, . This is the most accurate measurement of the exclusive . The result, is compatible with the theoretical value, 8.51 × 10(−)(8), calculated using the Chiral Perturbation Theory framework at next-to-leading order.